Thanks for reaching out to Ask Lafeber. A cockatiel that suddenly flew backward, crashed into objects, and now cries out in pain when attempting to fly is concerning. Unfortunately, it is impossible to determine whether the wing is broken, sprained, dislocated, or badly bruised without a veterinary examination. However, the fact that he crashed, can no longer fly properly, holds the wing up abnormally, and vocalizes in pain when trying to use it suggests that the injury may be more serious than a simple bruise. Wing fractures, sprains, and soft tissue injuries can all cause similar signs, including pain, reluctance to move the wing, and loss of flight ability. For now, it is important to prevent him from attempting to fly by keeping him in a smaller cage or carrier with food and water easily accessible and limiting his activity as much as possible. Monitor him closely for signs such as a drooping wing, swelling, difficulty perching, labored breathing, sitting on the cage floor, or decreased appetite, as these would warrant immediate veterinary attention. Because birds often hide injuries well and he is showing obvious pain when trying to fly, I would treat this as a potentially serious wing injury and recommend having him examined by an avian veterinarian as soon as possible. In the meantime, strict rest is the safest course of action.